The present invention relates to a method and apparatus for detecting the position of an object with an ultrasonic wave. More particularly, it relates to a method and apparatus of the type described which are well-suited for sensing the position of an obstacle that lies in a dead angle area around an automobile as viewed from the driver.
The detection of the position of an object has been performed by transmitting an ultrasonic wave into the air in the shape of pulses and measuring the period of time taken until the ultrasonic wave reflected by the object returns to a receiver. Part of the transmitted ultrasonic wave is received by the receiver directly without being reflected by the object. Such received wave, namely, a side-lobe wave is independent of the position of the object. Therefore, only the reflected wave is usually specified in such a way that a false signal simulative of the side-lobe wave is separately generated and that a component corresponding to the side-lobe wave is removed from the received wave by comparison. Such position-sensing apparatus is disclosed in Japanese Laid-open Patent Application No. 51-105851.
The prior-art position-sensing apparatus stated above has the problem that, if an object exists at a very close distance, the reflected wave is received in superposition on the side-lobe wave, so the position of the object cannot be accurately detected in consequence of the removal of the side-lobe wave. That is, an error is involved in the detection of the object within a distance corresponding to a time width in which the side-lobe wave affects the reception waveform. To the end of enhancing the detection accuracy of an object at a close distance, it is considered to merely shorten the transmission time (pulse width) of an ultrasonic wave to be transmitted, thereby to reduce the time width in which a side-lobe wave is detected. Since, however, an ultrasonic wave transmitter requires a certain period of time before the sound pressure of an acoustic wave reaches a necessary pressure level, the transmission pulse width cannot be shortened beyond a certain limit. Moreover, in the case the pulse width is set to be smaller than a certain value, a sufficient sound pressure cannot be attained, and the detection of an object at a far distance becomes impossible. Therefore, such measure is not practicable.